1. Field of the Invention
The present invention relates to a method and device for generating image data, which involve receiving input of image data and applying image density conversion processing to the image data, as well as a stencil printing apparatus.
2. Description of the Related Art
Various stencil printing apparatuses have been proposed. In stencil printing, a master sheet is produced through plate making processing using a thermal head, or the like, which is driven to melt and perforate a stencil master sheet according to image data obtained by reading an original document with a scanner, or the like. Then, the thus produced master sheet is wrapped around a printer drum and an ink is fed from the interior of the printer drum. The ink is transferred onto a printing sheet using a roller, or the like, to achieve printing.
Among the stencil printing apparatuses as described above, stencil printing apparatuses that perform double-face printing and that perform two-color printing, for example, have been proposed.
With the stencil printing apparatuses as described above, problem of contamination of prints may occur due to so-called “strike through”, “set-off”, retransfer of ink, etc., when the ink is excessively transferred onto the printing sheet, for example. In particular, with a stencil printing apparatus for double-face printing, for example, which includes a printer drum used for printing with a first master sheet and a printer drum used for printing with a second master sheet, the printing with the second master sheet is carried out before the ink from the first master sheet has sufficiently been dried. Therefore, the ink on the printing sheet tends to be transferred onto conveyance rollers and pressing rollers disposed along the sheet feeding path, and be further transferred from the rollers to another printing sheet.
In order to solve this problem, a method has been proposed, which involves reducing the pressing pressure to reduce the amount of ink transfer, thereby minimizing the retransfer of ink.
Further, Japanese Unexamined Patent Publication No. 2006-315288 (which is hereinafter referred to as “Patent Document 1”) has proposed a method for achieving an optimal amount of ink transfer by controlling plate making conditions, such as plate making energy and dot density, according to an image feature quantity of each of small areas of an image to be formed on the stencil master sheet through plate making.
However, with the above-described method that involves reducing the pressing pressure, although the amount of ink transfer can be reduced, density stability is impaired and the image quality is degraded if the pressing pressure is excessively low.
Further, even when the amount of ink transfer is controlled in the manner as in the method disclosed in Patent Document 1, the printing pressure applied from the pressing roller to the printer drum varies when printing sheets having different thicknesses and widths are used, and thus an optimal amount of ink transfer may not necessarily be achieved.
For example, when an A3-size printing sheet is used, a relatively large contact area is provided between the pressing roller and the printing sheet, and thus a uniform printing pressure is applied to the printing sheet. On the other hand, when an A4-size printing sheet is used, a smaller contact area is provided between the pressing roller and the printing sheet, and thus a higher printing pressure is applied to the printing sheet. Further, since the distance between the printer drum and the pressing roller is fixed, a higher printing pressure is applied to a thicker printing sheet, and a lower printing pressure is applied to a thinner printing sheet.
Therefore, when conditions of the printing sheets vary, it is impossible to achieve an optimal amount of ink transfer, and this may result in prints with insufficient density or prints with contamination due to the ink retransfer.